Is Shadow Blast bugged?

[Aleph]

You might also consider Plague if you have a lot of undead troops, or Banefire for those who have easy access (although I usually find that, save for sieges, the Decay effect only really kicks in when people are starting to run anyway - but it's still nice not to have to face them later).

[Shovah32]

The decay effect of banefire is nice but its the huge damage that makes it good imo

[Aleph]

Yeah, it definitely kills whoever you aim it at unless it's a pretender, which was the basis for my recommendation - should probably have made that explicit.